Instruments for optical measurement of distances are of great importance in manufacturing processes which require precise distance measurements. They are being used in the manufacture of automobiles, airplanes, and other goods. These measurement instruments use laser light sources and often require precise knowledge of emission frequency.
Several gage tools which use interferometric techniques have been proposed. An example of one using two-wavelength interferometry is found in Williams et al., "Absolute Optical Ranging with 200 nm Resolution," Optical Letters, vol. 14, no. 11, pp. 542-544. The proposed tools require precise knowledge of the frequency separation of light from two laser sources, typically with a rapid update, absolute measurement accuracies on the order of 0.05 GHz, and an operational range of over 150 GHz. In some cases, the measurement must be made while the lasers are being rapidly tuned in frequency at rates greater than 250 GHz per second. The instrument for performing this measurement should be compact, inexpensive, and ruggedly packaged for use on the factory floor.
The object of the invention, therefore, is to provide a compact, inexpensive means for determining the frequency separation between two monochromatic sources with rapidity and a high degree of accuracy, capable of being ruggedized for use in practical applications such as manufacturing.
Such a device would also have application for absolute frequency measurement when calibrated against a known frequency, and to analysis of multiple spectral lines, where such lines are sequentially analyzed.